Controlling the area where solder flows and limiting the bonding area during die attach is very difficult. Generally, the entire die is bonded, which results in excessive stress to the die. This can be very detrimental to the performance of a die such as a micro-electro-mechanical systems (MEMS) sensor.
In general, a small solder area is all that is required to meet strength requirements for die attach. High temperature solders are usually extremely strong, and have the desirable attribute of high material elasticity, which is critical to the predictable performance of MEMS sensors. When using solders for die attach, it can be difficult to control the final area, shape, and location of the solder joint.
The most common solution for attaching die is to use a sufficient volume of solder to completely cover the back side of the die, and to overflow next to the die. While this approach can make for a solid joint, it also imparts great stress on the attached die because of the mismatch of the coefficient of thermal expansion (CTE) of the materials used, including the die material, the substrate or package to which the die is mounted, and the solder itself For example, silicon die have a CTE in the range of about 2 to 3 ppm/° C. Ceramics to which die are often mounted, such as alumina, have a higher CTE, typically around 7 ppm/° C., and printed circuit boards have a much higher CTE.
Die stress can be reduced greatly by controlling the area over which a die is soldered. A small area near the center is usually best, but this can depend on the design and purpose of the die, especially when dealing with MEMS sensors. Unfortunately, die solders are notoriously difficult to control as the solders wet unpredictably, and voiding and flow are hard to predict. Simply placing a small amount of solder near the center of a die and reflowing can result in almost any shape and thickness of final solder area, even when conditions are carefully controlled. Other methods of die attach such as pure gold stud bump (GSB) thermocompression can control the bond area, but suffer from the soft, inelastic nature of the stud bump material.